Apparatus of charged-particle beam such as electron microscope comprising plasma generator, and method thereof

ABSTRACT

The present invention provides an apparatus of charged-particle beam e.g. an electron microscope comprising an in-column plasma generator for selectively cleaning BSE detector and BF/DF detector. In various embodiments, the plasma generator is located between a lower pole piece of objective lens and the BF/DF detectors, but outside trajectory area of the charged-particles from the sample stage to the BF/DF detector. Cleaning decomposed biological samples or contaminants on the surface of the detectors frequently and selectively with in-situ generated plasma can prevent the detectors from performance deterioration such as losing resolution and contrast in imaging at high levels of magnification.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is Continuation-in-Part of U.S. non-provisionalapplication Ser. No. 17/444,192 filed on Aug. 1, 2021 and docketed as“Elastic Connector,” which claims the benefit under 35 U.S.C. Section119(e) and Article 4 of the Stockholm Act of the Paris Convention forthe Protection of Industrial Property of U.S. Provisional PatentApplication No. 63/087,238, filed Oct. 4, 2020, entitled “SeveralDesigns for Apparatus of Charged-Particle Beam and Methods Thereof,” allof which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention generally relates to a local plasma generator, andan apparatus of charged-particle beam such as electron microscope usingthe same for electively cleaning one or more parts in the apparatus.Although the invention will be illustrated, explained, and exemplifiedby electron microscopes, it should be appreciated that the presentinvention can also be applied to other fields, for example, electronbeam recorders, electron beam lithography systems, and the like.

BACKGROUND OF THE INVENTION

Owing to the small de Broglie wavelength of electrons, TEM and STEM canenable the user to examine fine detail as small as a single column ofatoms. Therefore, electron microscopes find application in cancerresearch, virology, materials science as well as pollution,nanotechnology, and semiconductor research, and they are used toinvestigate the ultrastructure of a wide range of specimens includingtumor cells, microorganisms, large molecules, biopsy samples,semiconductor device, metals, and crystals.

For example, most viruses can be seen only by TEM (transmission electronmicroscopy), since light microscopes are limited by light itself as theycannot show anything smaller than half the wavelength of visible light,and viruses are much smaller than this. For instance, naked viruses areicosahedral, and their protein coat or capsid is more rigid andwithstands a drying process well to maintain their spherical structurein negative stains. Naked human viruses are of three size ranges: 22 to35 nm (e.g., parvoviruses, enteroviruses, and caliciviruses), 40 to 55nm (polyomaviruses and papillomaviruses), and 70 to 90 nm (reoviruses,rotaviruses, and adenoviruses).

TEM has therefore made a major contribution to virology, including thediscovery of many viruses, the diagnosis of various viral infections andfundamental investigations of virus-host cell interactions. TEM is veryuseful for the initial identification of unknown viral agents inparticular outbreaks, and it is recommended by regulatory agencies forinvestigation of the viral safety of biological products and/or thecells used to produce them. In research, only TEM has a resolutionsufficiently high for discrimination between aggregated viral proteinsand structured viral particles. Additionally, the fine detail of viralstructure may become visible if viral preparations are rapidly frozenand the vitrified specimens examined by cryo-EM. When combined with datafrom X-ray diffraction studies, or with electron tomography orsingle-particle analyses of isolated virions, highly detailed structurescan be obtained at near atomic resolution.

Cynthia S. Goldsmith et al. have reported in Clinical MicrobiologyReviews, Vol. 22, No. 4, October 2009, p. 552-563 “Modern Uses ofElectron Microscopy for Detection of Viruses” that electron microscopy(EM) is still on the forefront of virus identification, particularly incases where agents are unknown or unsuspected. EM is a valuabletechnique in the surveillance of emerging diseases and potentialbioterrorism agents. Methods for treatment of or vaccination againstviral diseases can be investigated with EM through ultrastructuralstudies that elucidate both viral makeup and the relationship of virusesto the cells they infect. In 1948, differences between the virus thatcauses smallpox and the virus that causes chickenpox were demonstratedby EM. The first image of poliovirus was taken in 1952, and virus-hostrelationships began to be studied in the mid-1950s. Early virusclassification depended heavily on morphology as shown by EM, and manyof the intestinal viruses were discovered by EM examination of fecesafter negative staining. One of the main advantages of using EM forviral diagnosis is that it does not require organism-specific reagentsfor recognizing the pathogenic agent. EM can sometimes elaborateultrastructural differences in the morphologies of similar viruses. Forexample, differences in Marburg and Ebola viruses, both of which are inthe filovirus family, have been demonstrated by EM. Marburg virusvirions are shorter than those of Ebola virus, and their surface spikesdiffer. Even when EM cannot identify a virus beyond the family level, itat least points the way for more specific identification by othermethods such as biochemical assays for specific pathogens. Virusesstored in various solutions for extended periods are not viable forculture detection and may be unsuitable for molecular testing. However,EM does not require live or intact virus. EM is on the front line insurveillance of viruses that might be used by terrorists. Therapid-response laboratories in the Laboratory Response Network arepaired with EM facilities across the United States.

Philippe Roingeard has reported in Biol Cell. 2008 August; 100 (8):491-501 “Viral detection by electron microscopy: past, present andfuture” that the benefits of TEM for resolving diagnostic problems inclinical virology have been clearly illustrated on several occasions.TEM proved essential for the identification of a new morbillivirus(Hendra virus, belonging to the Paramyxoviridae) in horses and humanssuffering from fatal respiratory infections in 1995 in Australia. Arelated virus, the Nipah virus, mostly affecting pig farmers inMalaysia, was discovered in 1990s. The etiology of the SARS (severeacute respiratory syndrome) pandemic in Hong Kong and Southern China in2003 was first identified as a coronavirus by TEM, leading to subsequentlaboratory and epidemiological investigations. A human monkeypoxoutbreak in the U.S.A. in 2003 was also diagnosed only once TEM had beenused. TEM is occasionally useful for the identification of new subtypesof viruses involved in human gastroenteritis, such as adenovirus orpicornavirus. The role of TEM in clinical virology becomes supportingthe identification of unknown infectious agents in particular outbreaks.In such investigations, the underlying ‘catch-all’ principle of thistechnique is essential for the recognition of an unknown agent. Thereare also many examples of the usefulness of TEM for identifying thevirus involved in particular outbreaks in veterinary medicine.

FIG. 1 is an illustration created at the Centers for Disease Control andPrevention (CDC), and it reveals ultrastructural morphology exhibited bycoronaviruses. The spikes that adorn the outer surface of the virusimpart the look of a corona surrounding the virion, when viewed electronmicroscopically. A novel coronavirus, named severe acute respiratorysyndrome coronavirus 2 (SARS-CoV-2), was identified as the cause of anoutbreak of respiratory illness first detected in Wuhan, China in 2019.The illness caused by this virus has been named coronavirus disease 2019(COVID-19). The symptoms of COVID-19 are highly variable, ranging fromnone to severe illness, and to death. Electron microscopy visualizationshows that SARS-CoV-2 particles have approximately 150-200 nanometers indiameter. Electron microscopy has been used to determine how theSARS-CoV-2 uses its outer “spike” protein to interact with human cellsand infect them. Such studies are really useful in working out how thevirus gains access to human cells so scientists can work out how to usedrugs to block it.

However, biological samples such as viral samples may cause someproblems when they are examined by an electron microscope. For example,the BSE detector and/or the BF/DF detector of the electron microscopemay be quickly contaminated with decomposed biological samples (e.g.contamination from hydrocarbons) because of their proximity to thesample stage. Consequently, the performance of the detectors isdeteriorated, for example, a loss of resolution and contrast in imagingat the highest levels of magnification. Despite using dry pumps andliquid nitrogen traps, a contamination haze continues to be formed inthe electron microscope. Advantageously, the present invention providesa new apparatus of charged-particle beam such as electron microscopewith a local plasma generator that can solve the problems.

SUMMARY OF THE INVENTION

One aspect of the present invention provides an apparatus ofcharged-particle beam comprising an in-column plasma generator. Invarious embodiments, the apparatus includes an objective lens, a samplestage, a BSE detector, a BF/DF detector, and a plasma generator. Theobjective lens comprises an upper pole piece and a lower pole piece, andthe sample stage is located between the two pieces. The BSE detector islocated above the sample stage, and the BF/DF detector is located belowthe lower pole piece. The plasma generator is located between the lowerpole piece and the BF/DF detector.

Another aspect of the present invention provides a method of selectivelycleaning the BSE detector and the BF/DF detector using the in-columnplasma generator as described above.

The above features and advantages and other features and advantages ofthe present invention are readily apparent from the following detaileddescription of the best modes for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements. All the figures areschematic and generally only show parts which are necessary in order toelucidate the invention. For simplicity and clarity of illustration,elements shown in the figures and discussed below have not necessarilybeen drawn to scale. Well-known structures and devices are shown insimplified form, omitted, or merely suggested, in order to avoidunnecessarily obscuring the present invention.

FIG. 1 shows an ultrastructural morphology of coronaviruses created atthe Centers for Disease Control and Prevention (CDC).

FIG. 2 schematically illustrates an apparatus of charged-particle beamcomprising a plasma generator in accordance with an exemplary embodimentof the present invention.

FIG. 3 schematically illustrates a plasma generator having a hollowcylindrical electrode in accordance with an exemplary embodiment of thepresent invention.

FIG. 4 schematically illustrates the position and configuration of theplasma generator having a hollow cylindrical electrode as relative to asample stage and a BF/DF detector within an apparatus ofcharged-particle beam in accordance with an exemplary embodiment of thepresent invention.

FIG. 5 schematically illustrates a plasma generator having a hollowcylindrical electrode around a dielectric cylinder in accordance with anexemplary embodiment of the present invention.

FIG. 6 schematically illustrates the position and configuration of theplasma generator having a hollow cylindrical electrode around adielectric cylinder as relative to a sample stage and a BF/DF detectorwithin an apparatus of charged-particle beam in accordance with anexemplary embodiment of the present invention.

FIG. 7 schematically illustrates some plasma generators useful in theapparatus of charged-particle beam in accordance with an exemplaryembodiment of the present invention.

FIG. 8 schematically illustrates another plasma generator useful in theapparatus of charged-particle beam in accordance with an exemplaryembodiment of the present invention.

FIG. 9 is a flow chart showing a method of selectively cleaning BSEdetector and BF/DF detector in an apparatus of charged-particle beam inaccordance with an exemplary embodiment of the present invention.

FIG. 10 schematically illustrates the formation of co-condensers whichcan be used in an apparatus of charged-particle beam with a plasmagenerator in accordance with an exemplary embodiment of the presentinvention.

FIG. 11 shows two co-condensers with magnetic coils which can be used inan apparatus of charged-particle beam with a plasma generator inaccordance with an exemplary embodiment of the present invention.

FIG. 12 illustrates an apparatus of charged-particle beam with amagnetic objective lens and a deflection system which can be used withthe plasma generator in accordance with an exemplary embodiment of thepresent invention.

FIG. 13 demonstrates a single large field of view (FOV) on the specimenplane of the apparatus in accordance with an exemplary embodiment of thepresent invention.

FIG. 14 demonstrates multiple large FOVs on the specimen plane of theapparatus in accordance with an exemplary embodiment of the presentinvention.

FIG. 15 illustrates a macroscopic deflection sub-system in accordancewith an exemplary embodiment of the present invention that alone causesthe beam to scan across a large FOV.

FIG. 16 illustrates a microscopic deflection sub-system causing the beamto scan across a small FOV in accordance with an exemplary embodiment ofthe present invention.

FIG. 17 schematically illustrates the configuration of a macroscopicdeflection sub-system which can be used with the plasma generator inaccordance with an exemplary embodiment of the present invention.

FIG. 18 schematically illustrates the configuration of a microscopicdeflection sub-system which can be used with the plasma generator inaccordance with an exemplary embodiment of the present invention.

FIG. 19 shows an apparatus of charged-particle beam with the plasmagenerator in accordance with an exemplary embodiment of the presentinvention.

FIG. 20 shows the image of a biological sample in a large FOV with lowresolution and a small FOV with high resolution in accordance with anexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It is apparent, however, to oneskilled in the art that the present invention may be practiced withoutthese specific details or with an equivalent arrangement.

Where a numerical range is disclosed herein, unless otherwise specified,such range is continuous, inclusive of both the minimum and maximumvalues of the range as well as every value between such minimum andmaximum values. Still further, where a range refers to integers, onlythe integers from the minimum value to and including the maximum valueof such range are included. In addition, where multiple ranges areprovided to describe a feature or characteristic, such ranges can becombined.

It is also to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and it is notintended to limit the scope of the invention. For example, when anelement is referred to as being “on”, “connected to”, or “coupled to”another element, it can be directly on, connected or coupled to theother element or intervening elements may be present. In contrast, whenan element is referred to as being “directly on”, “directly connectedto”, or “directly coupled to” another element, there are no interveningelements present.

With reference to FIG. 2, various embodiments of the invention providean apparatus 1 of charged-particle beam comprising an objective lens 6,a sample stage or a specimen holder 9, a BSE or SE detector 15, a BFdetector 16 and/or a DF detector 17 (hereinafter BF/DF detector 16-17),and a plasma generator 21. The objective lens 6 comprises an upper polepiece 6 a and a lower pole piece 6 b, and the sample stage 9 is locatedbetween the upper pole piece 6 a and the lower pole piece 6 b. The BSEdetector 15 is located above the sample stage 9 and the BF/DF detector16-17 is located below the lower pole piece 6 a. The plasma generator 21is located between the lower pole piece 61 and the BF/DF detector 16-17.The plasma generator 21 is so configured that it distributes ordissipates the plasma on surfaces of the BSE detector 15, the BF/DFdetector 16-17, the lower pole piece 6 b and the sample stage 9 inconcentrations that are higher than that on surface of any othercomponent(s) within the apparatus of charged-particle beam 1, forexample, a device 14P with a limiting aperture 14 that is located abovethe objective lens 6.

In various embodiments of the invention, the plasmas are made up of gasatoms in which some or all of the electrons have been stripped away(ionization) and positively charged nuclei (ions) roam freely or not,e.g. under the influence of an electric field (if any). Additionally,moving charged particles generate electric currents, and any movement ofa charged plasma particle affects and is affected by the fields createdby the other charges. In turn this governs collective behavior with manydegrees of variation. The plasma used in the present invention may benearly fully ionized (i.e. “hot”) or partially ionized (i.e. “cold”),where only a small fraction (e.g. 1%) of the gas molecules are ionized.The gas material transforms from being an insulator into a conductor, asit becomes increasingly ionized.

In preferred embodiments, the plasma generator 21 is configured toperiodically generate and distribute the plasma for selectively cleaningcontaminants on the surface of the BSE detector 15 and the BF/DFdetector 16-17. Plasma cleaning in the invention is a process ofremoving some or all organic matter from the surface of the detectorsthrough chemical reaction or physical ablation of e.g. hydrocarbons, toform gaseous products. If desired, the gaseous products may be sweptaway by a continuous gas flow. Sometimes, this may be performed in avacuum chamber utilizing oxygen and/or argon gas. The cleaning processof the invention is an environmentally safe process as there are noharsh chemicals involved.

In some embodiments, the BSE detector 15 and the BF/DF detector 16-17may have a voltage of 10-25V (e.g. a negative voltage) to attract andconcentrate the plasma onto their surfaces for more efficient andselective cleaning.

In preferred embodiments as shown in FIG. 2, the plasma generator 21 maybe configured not to block (or hinder) any charged-particles fromirradiating on the BF/DF detector 16-17. In other words, the plasmagenerator 21 is configured not to block (or hinder) anycharged-particles that would have been detected by the BF/DF detector16-17 in the absence of the plasma generator 21. It may be physicallylocated outside trajectory area T of the charged-particles from thesample stage 9 to the BF/DF detector 16-17. As shown in FIG. 2, thelower pole piece 6 b may have a recess 6 c, and the plasma generator 21is partially or completely located within the recess 6 c.

In preferred embodiments as shown in FIG. 3, the plasma generator 21comprises a source of radio-frequency electrical power (RF), a hollowcylindrical electrode 22 formed of conducting material, and a groundedshield 23 surrounding and enclosing the cylindrical electrode 22 andelectrically insulated therefrom. The cylindrical electrode 22 is incommunication with the source of radio-frequency electrical power (RF).Upon energizing the electrode 22 with a radio-frequency electric power,a plasma is generated from gas in an interior of the cylindricalelectrode 22. As shown in FIG. 4, a central hallow space (or theinterior) of the cylindrical electrode 22 is so positioned anddimensioned that it allows charged-particles to travel through it fromthe sample stage 9 to the BF/DF detector 16-17. It does not block (orhinder) any charged-particles from irradiating on the BF/DF detector16-17. In other words, it does not block (or hinder) anycharged-particles that would have been detected by the BF/DF detector16-17 in the absence of the plasma generator 21.

In other preferred embodiments as shown in FIG. 5, the plasma generator21 further comprises a hollow dielectric cylinder 24 formed of adielectric material. The cylindrical electrode 22 may be a brasscylinder around an exterior diameter of the dielectric cylinder 24. Thecylindrical electrode 22 is surrounding an exterior of the dielectriccylinder 24. Upon energizing the cylindrical electrode 22 withradio-frequency electric power, a plasma is generated from gas in aninterior of the dielectric cylinder 24 by radio-frequency, hollowcathode effect coupling inside the dielectric cylinder 24. As shown inFIG. 6, a central hallow space (or the interior) of the dielectriccylinder 24 is so positioned and dimensioned that it allowscharged-particles to travel through it from the sample stage 9 to theBF/DF detector 16-17. Put differently, it does not block (or hinder) anycharged-particles from irradiating on the BF/DF detector 16-17. In otherwords, the dielectric cylinder 24 does not block (or hinder) anycharged-particles that would have been detected by the BF/DF detector16-17 in the absence of the plasma generator 21. In some exemplaryembodiments, the dielectric cylinder 24 is formed of ceramic, glass,quartz, and Teflon such as machinable ceramic comprising about 55%fluorophlogopite mica and 45% borosilicate glass. Without being bound byany particular theory, a virtual anode may be formed by the hollowcathode effect along a central axis of the dielectric cylinder 24 in theplasma and a ground may be defined by the B SE detector 15 and/or theBF/DF detector 16-17.

In the apparatus of the present invention, a source of gas may beprovided for producing the plasma. For example, a vacuum chamber may becontrolled to lower its vacuum pressure, allowing a desired amount ofambient air to flow into the chamber and then the air is used forgenerating plasma. Alternatively, a source of gas (e.g. a tank) may bein fluid communication with the interior of the cylindrical electrode 22as shown FIG. 3, or the interior of the dielectric cylinder 24 as shownFIG. 5, through a gas flow control device.

It should be appreciated that any other suitable plasma generator(s) maybe employed in the present invention for generating the plasma forcleaning detectors. The type of power source used to generate the plasmaof the invention may be DC, AC (typically with radio frequency, but notnecessarily limited thereto) and microwave. The plasma may be generatedby the application of an electric field, a magnetic field, a microwave,or any combination thereof through a gas selected from oxygen, nitrogen,air, hydrogen, argon, helium, and neon. The pressure for plasmaoperation may be vacuum pressure (<10 mTorr or 1 Pa), low or moderatepressure (<1 Torr or 100 Pa), or atmospheric pressure (< or =760 Torr or100 kPa), preferably low or moderate pressure. The temperaturerelationships within the plasma may be thermal plasma, non-thermal orcold plasma.

Any known mechanism(s) may be employed in the present invention forgenerating the plasma. Examples of low-pressure plasma discharge includeglow discharge, capacitively coupled plasma (CCP), cascaded arcdischarge, inductively coupled plasma (ICP), and wave heated plasma,among others. Glow discharge plasma is non-thermal plasmas generated bythe application of DC or low frequency RF (<100 kHz) electric field tothe gap between two metal electrodes. Capacitively coupled plasma (CCP)is similar to glow discharge plasmas, but it is generated with highfrequency RF electric fields, typically 13.56 MHz. Any known CCP devicesused in the microfabrication and integrated circuit manufacturingindustries for plasma etching and plasma enhanced chemical vapordeposition may be properly modified and used in the present invention.Cascaded arc plasma source is a device to produce low temperature (≈1eV) high density plasmas (HDP), and it may also be properly modified andused in the present invention. Inductively coupled plasma (ICP) issimilar to CCP except that the electrode consists of a coil wrappedaround a chamber where plasma is formed. Wave heated plasma is similarto CCP and ICP in that it is typically RF (or microwave). Examplesinclude helicon discharge and electron cyclotron resonance (ECR), andmagnetically induced plasmas (MIP) which is typically produced usingmicrowaves as a resonant coupling method.

For example, an inductively coupled plasma (ICP) or transformer coupledplasma (TCP) as shown in FIGS. 7-8 may be used in the present invention.The energy is supplied by electric currents which are produced byelectromagnetic induction (time-varying magnetic fields). FIG. 7 shows aplanar ICP geometry in which the electrode is a length of flat metalwound like a spiral (or coil), and a cylindrical ICP geometry in whichit is like a helical spring. FIG. 8 shows a half-toroidal ICP geometry,in which the electrode is toroidal solenoid cut along its main diameterto two equal halves. When a time-varying electric current is passedthrough the coil, the ICP creates a time-varying magnetic field aroundit. According to the Faraday-Lenz's law of induction, this createsazimuthal electromotive force in the rarefied gas to generate theplasma. In an embodiment, the ICP uses a radio-frequency-powered (RF)solenoid magnetic field to trap the electrons in the plasma. Because theICP coil is grounded on one leg, it has low impedance and high currentwith high heating. The electron energy spread expands due to collidingand circling electrons in the magnetic field without encounters with thesheath. A solenoid coil can be wrapped around a glass or quartz cylinderand excited with RF power to create an inductively coupled plasma (ICP)without the electrode contacting the plasma. The ICP creates a magneticfield within the chamber that traps the free electrons within the coilwhile they oscillate.

Referring back to FIGS. 3-6, the RF plasma source 21 can be used tocreate ions, electrons, excited metastables, and atomic radicalsdepending on choices of gas, pressure, flow rates, RF power andfrequency, and extraction electrodes. In certain embodiments, the gasmay be air and the detectors 15, 16 and 17 are cleaned by oxygenradicals that remove carbon compounds by oxidation. In otherembodiments, the gas may be hydrogen. As a secondary benefit, RF plasmasource 21 can also be used to clean other contaminations on other partsof the apparatus 1 of charged-particle beam, such as pump oils,fingerprints, dirty specimens, and improper vacuum practices inmanufacturing and operation. Plasma cleaning with an air plasma removeshydrocarbons with a chemical etch where the oxygen in air isdisassociated into neutral O radicals (atoms) or metastables. Thesespecies react quickly with hydrocarbons to produce H₂O, CO₂, CO, H₂CO,and other short chain volatile hydrocarbons that can be removed by thevacuum pumps. Hydrogen gas can also be used in a plasma for cleaning byreduction of the hydrocarbons. Other frequently used gases includecombinations of N₂, O₂, H₂, fluorocarbons, as well as inert dilutantgasses He, Ne, Ar, Ne, and Xe.

The radio-frequency-excited hollow cathode (RF-HC) 22 may become aplasma radical source and it can create an excited gas plasma inside theapparatus 1 of charged-particle beam, such as a vacuum chamber in anelectron microscope. The hollow cathode 22 may be made from an aluminumscreen by machining or from punched sheet metal. Electrode 22 may alsobe “assembled” of halves or quarters machined or otherwise formed fromelectrically conductive metals or other conductive materials that areclosely joined together, electrically, physically, or structurally, uponassembly. Electrode 22 may be a continuous thin conductive cylinder, asdistinguished from a coil or other interrupted structure, to avoidinductive coupling effects. Other conductive materials such as aluminumor copper could also be used for the electrode. A cylindrical,electrically conductive shield 23 may be placed around the electrode 22and is electrically grounded. Shield 23 may be grounded by a connectionto the shield of a RF cable (not shown). An insulator in the form of anair gap (as shown), or solid dielectric material (not shown), may beused to separate electrode 22 from shield 23.

In an embodiment, an aluminum cylindrical hollow cathode 22 as shown inFIGS. 3-4 is immersed in the plasma during operation. At low power (<20Watts RF @13.56 MHz) this may work well and is particularly preferred.However, at a higher power, overheating and electrode erosion occurredand discoloration formed on the interior walls of the plasma source asshown in FIGS. 3-4. This may suggest material losses from the electrode22 and its support structure. The RF power may be fed through a powerfeedthrough on a flange (not shown) which supports the electrode 22 onits axis via a support cross bar (not shown).

In other embodiments, the plasma generator 21 as shown in FIGS. 5-6 aremore preferred, since the design enables low voltage, high currentoperation to prevent overheating, erosion of the electrode, andparticulate generation. Due to the dielectric cylinder, this designremoves the conductive material of the electrode 22 from contacting withions from the plasma. The plasma chamber and electrode 22 can be mountedinside an outer grounded shell 23 for electrical safety. The reactantgas may be air because it is a convenient source of oxygen. Other oxygengas mixtures and pure oxygen can be used or reducing gas can be used.These mixtures can contain hydrogen, water vapor, He, Ar, Ne, F andcompounds thereof. For cleaning by reduction H₂ and ammonia could beused. With an exterior hollow cathode 22, the dielectric cylinder 24will partially enclose and define a plasma sheath. Ions will createsecondary electrons when they collide with the dielectric material andthe expelled ions will be accelerated into the plasma by the sheath.Inside the plasma the high energy ions are very effective in ionizationand disassociation of the gas molecules.

Although atmospheric pressure plasma can be used, in the invention, itis less preferred. Examples include arc discharge, corona discharge,dielectric barrier discharge (DBD), capacitive discharge, andpiezoelectric direct discharge plasma. Corona discharge is a non-thermaldischarge generated by the application of high voltage to sharpelectrode tips. Dielectric barrier discharge (DBD) is a non-thermaldischarge generated by the application of high voltages across smallgaps wherein a non-conducting coating prevents the transition of theplasma discharge into an arc. Capacitive discharge is a nonthermalplasma generated by the application of RF power (e.g., 13.56 MHz) to onepowered electrode, with a grounded electrode held at a small separationdistance on the order of 1 cm. Such discharges are commonly stabilizedusing a noble gas such as helium or argon. Piezoelectric directdischarge plasma is a nonthermal plasma generated at the high-side of apiezoelectric transformer (PT).

Another aspect of the present invention provides a method of selectivelycleaning BSE detector 15 and BF/DF detector 16-17 in an apparatus 1 ofcharged-particle beam, as shown in FIG. 9. The method includesinstalling a plasma generator 21 within the apparatus 1 in a manner asdescribed above; and generating plasma to selectively clean the BSEdetector 15 and/or the BF/DF detector 16-17.

In some embodiments, the apparatus of charged-particle beam 1 mayinclude a charged-particle optical column and a sample chamber. Forexample, the charged-particle optical column may include one or morecharged-particle optical components along the beam path, selected from asource of charged particles configured to emit a beam of chargedparticles such as an electron gun 2 configured to emit an electron beam,condenser(s), stigmator(s), alignment coil(s), alignment plate(s), beamblanking(s), plate(s) 14P with objective (or limiting) aperture(s) 14,plate(s) with spread aperture(s), deflector(s), magnetic objectivelens(es), and detector(s). The sample chamber may include one or morechamber components selected from a specimen holder 9 for holding aspecimen under examination, a receptacle for receiving a lithographicalworkpiece (e.g. mask or wafer) being processed with the beam, and one ormore detectors for detecting charged particles (such as BSE detector 15and BF/DF detector 16-17 as described above). As shown in FIG. 19,charged-particle optical components within the column may be electronoptical components selected from the following (from upstream todownstream): an electron gun 2 configured to emit an electron beam, afirst co-condenser 3, a second co-condenser 4, a beam blanking 13, aplate 14P with an objective aperture 14, a stigmator 71 s, an uppermacroscopic deflector 71 a, an upper microscopic deflector 72 a, a lowermicroscopic deflector 72 b, a lower macroscopic deflector 71 b, amagnetic objective lens 6, and a BSE or SE detector 15.

In preferred embodiments, the apparatus of charged-particle beam 1 is anelectron microscope (such as STEM), or an electron beam lithographyapparatus. In the following exemplary embodiments, the plasma generator21 of the invention is used to optimize an apparatus of charged-particlebeam 1 having con-condensers as shown in FIGS. 10-11.

In an apparatus of charged-particle beam such as an electron microscope(e.g. STEM), the manipulation of an electron beam is performed using twophysical effects. The interaction of electrons with a magnetic fieldwill cause electrons to move according to the left-hand rule, thusallowing for electromagnets to manipulate the electron beam. The use ofmagnetic fields allows for the formation of a magnetic lens of variablefocusing power, and the lens shape is determined by the distribution ofmagnetic flux. Electrostatic fields can cause the electrons to bedeflected through a constant angle. Coupling of two deflections inopposing directions with a small intermediate gap allows for theformation of a shift in the beam path. From these two effects, as wellas the use of an electron imaging system, sufficient control over thebeam path is made possible. The lenses in the beam path can be enabled,tuned, and disabled entirely and simply via rapid electrical switching,the speed of which is only limited by effects such as the magnetichysteresis.

In an apparatus 1 of charged-particle beam as shown in FIG. 10, a source2 of charged particles is configured to emit a beam of chargedparticles. The source 2 may be for example an electron gun with atungsten filament or a lanthanum hexaboride (LaB₆). In panel (a), amagnetic condenser 3 alone can focus the beam to a crossover spot F1.The beam is expanded after a crossover spot. In panel (b), anothermagnetic condenser 4 is placed between magnetic condenser 3 andcrossover spot F1, and the beam is now focused to a new crossover spotF2 that is closer to source 2 than spot F1. In panel (c), a thirdmagnetic condenser 5 is placed between magnetic condenser 4 andcrossover spot F2, and the beam is again focused to another newcrossover spot F3 that is even closer to source 2 than spot F2.

Generally, a condenser lens forms an image of the primary electron beamsource for an objective lens, and the objective lens focuses thecondenser lens image onto a specimen. Transmitted, secondary andbackscattered electrons are released from the specimen material. Theseelectrons are detected, amplified and the resulting signal used tomodulate the beam of an imaging system operating synchronously with thescanning electron beam. The result is an image of the scanned area basedon the electrons emitted or scattered from the specimen.

In the present invention, the term “co-condensers” is defined as two ormore magnetic condensers configured to coherently focus the beam to asingle crossover spot F. For example, magnetic condensers 3 and 4 inpanel (b) coherently focus the beam to a single crossover spot F2, andthey may be called a set of co-condensers. Magnetic condensers 3, 4 and5 in panel (c) coherently focus the beam to a single crossover spot F3,and they may also be called a set of co-condensers. As shown in FIG. 10,the beam does not have any crossover spot between any two of the two ormore magnetic condensers within a set of co-condensers.

The crossover spot F may be movable or immovable. In some embodiments ofthe invention, the single crossover spot F is so controlled that itremains stationary or immovable relative to the source 2 of chargedparticles. For example, crossover spot F2 may be kept stationaryrelative to the source 2, i.e. the distance D0 between spot F2 andsource 2 remains unchanged. By the same token, crossover spot F3 may bekept stationary relative to the source 2, i.e. the distance D0 betweenspot F3 and source 2 remains unchanged.

While the single crossover spot F remains immovable relative to thesource 2 of charged particles, the size A of the beam at crossover spotF (i.e. the cross-sectional area of the beam at F) may be so controlledto have a desired value. Preferably, size A may be tuned/adjusted byconcertedly tuning/adjusting the individual condensing capacity of thetwo or more magnetic condensers within a set of co-condensers. Forexample, the condensing capacity of condenser 3 and that of condenser 4may be individually but concertedly tuned/adjusted so that not only thesingle crossover spot F2 is fixed relative to the source 2, but also thesize A of the beam at crossover spot F2 is controlled to have a value asdesired. Likewise, the condensing capacities of two or more condensers3, 4 and 5 may be individually but concertedly tuned/adjusted so thatnot only the single crossover spot F3 is fixed relative to the source 2,but also the size A of the beam at crossover spot F3 is controlled tohave a value as desired. The two or more co-condensers are thereforeconfigured to coherently focus the beam to the same cross-over pointwith different magnification rates.

Although the apparatus 1 may include one, two or more sets ofco-condensers, in some preferred embodiments of the invention, theapparatus 1 includes only one set of co-condensers with only twomagnetic condensers configured to coherently focus the beam to a singlecrossover spot F. For example, the apparatus 1 may include only one setof co-condensers as shown in Panel (b) of FIG. 10 with only two magneticcondensers (3, 4) configured to coherently focus the beam to a singlecrossover spot F2.

Referring to FIG. 11, the only two magnetic condensers (3, 4) include adistal magnetic condenser 4 which is distal to the source 2, and aproximal magnetic condenser 3 that is located between the source 2 andthe distal magnetic condenser 4. The proximal magnetic condenser 3comprises a magnetic coil 3C driven by a coil current I1; and the distalmagnetic condenser 4 comprises a magnetic coil 4C driven by a coilcurrent I2. Generally, both coil currents I1 and I2 are greater than 0(>0).

In preferred embodiments of the invention, coil currents I1 and I2 areconfigured to position single crossover spot F2 at a fixed position,i.e. maintain a predetermined distance D0 from source 2. With the “FixedF2” condition being met, the size A of the crossover spot F2 may beincreased by increasing coil current I1 and/or decreasing coil currentI2; and decreased by decreasing coil current I1 and/or increasing coilcurrent I2. The size A of the crossover spot F2 will be minimized whencoil current I1 reaches its minimal value while coil current I2 reachesits maximal value. The size A is maximized when coil current I2 reachesits minimal value while I1 reaches its maximal value. There is nospecial limitation on the maximized size A, it may be smaller than,equal to, or bigger than the size of the source 2.

In various exemplary embodiments as shown in FIG. 12, the apparatus ofcharged-particle beam according to the invention may include a magneticobjective lens 6 (as shown in FIGS. 17-19) and a deflection system 7,both of which are downstream with respect to the single crossover spot F(e.g. F2). Although electron lenses may operate electrostatically ormagnetically, most electron lenses use electromagnetic coils to generatea convex lens. The field produced for the lens is typically radiallysymmetrical, as deviation from the radial symmetry of the magnetic lenscauses aberrations such as astigmatism and worsens spherical andchromatic aberration. For example, a quadrupole lens is an arrangementof electromagnetic coils at the vertices of the square, enabling thegeneration of a lensing magnetic fields, the hexapole configurationsimply enhances the lens symmetry by using six, rather than four coils.Electron lenses may be manufactured from iron, iron-cobalt or nickelcobalt alloys, such as permalloy, due to their good magnetic properties,such as magnetic saturation, hysteresis and permeability. It should beappreciated that the objective lens 6 may be an electromagnetic lens oran electrostatic lens.

Objective lens 6 allows for electron beam convergence, with the angle ofconvergence as a variable parameter. The magnification may be simplychanged by modifying the amount of current that flows through the coilof lenses. Lens 6 may include yoke, magnetic coil, poles, pole piece,and external control circuitry. An electromagnetic lens 6 may include anupper pole piece 6 a and a lower pole piece 6 b, as described above. Thepole piece must be manufactured in a very symmetrical manner, as thisprovides the boundary conditions for the magnetic field that forms thelens. Imperfections in the manufacture of the pole piece can inducesevere distortions in the magnetic field symmetry, which inducedistortions that will ultimately limit the lenses' ability to reproducethe object plane. The exact dimensions of the gap, pole piece internaldiameter and taper, as well as the overall design of the lens is oftenperformed by finite element analysis of the magnetic field, taking intoaccount of the thermal and electrical constraints of the design. Thecoils which produce the magnetic field are located within the lens yoke.The coils can contain a variable current, but typically utilize highvoltages, and therefore require significant insulation in order toprevent short-circuiting the lens components. Thermal distributors areplaced to ensure the extraction of the heat generated by the energy lostto resistance of the coil windings. The windings may be water-cooled,using a chilled water supply in order to facilitate the removal of thehigh thermal duty.

A magnetic lens may include a magnetic material and exciting coils forproviding magnetomotive force to a magnetic circuit having field linesthrough the magnetic material and between pole faces.

For the deflection system 7, it may include a macroscopic deflectionsub-system 71 and a microscopic deflection sub-system 72. The deflectionsystem 7 causes the beam to position at, and scan across, a large fieldof view (FOV) on a specimen plane 8 of a specimen under examination in aspecimen holder 9 and one or more small FOVs within the large FOV.

As shown in FIGS. 13 and 14, the macroscopic deflection sub-system 71causes the beam to scan across a large field of view (FOV) 10 on thespecimen plane 8 of the specimen holder 9, and the microscopicsub-deflection system 72 causes the beam to position at, and scanacross, one or more small FOVs 11 within a large FOV. As shown in FIG.13, the specimen plane 8 may contain only one large FOV 10, which maycontain zero, one, two, three or more small FOVs 11. In FIG. 14, thespecimen plane 8 may contain two, three or more large FOVs 10, each ofwhich may contain zero, one, two, three or more small FOVs 11.

In the first step of an examination process as shown in FIG. 15, a usermay turn off or inactivate the microscopic sub-deflection system 72.Then, the macroscopic deflection sub-system 71 causes the beam to scanacross a large FOV 10 on the specimen plane 8 of the specimen holder 9under a lower resolution (e.g. 10 nm). After the large FOV scanning iscompleted, the user finds a pattern of interesting (POI) in one or moresmall FOVs 11 within that large FOV 10, and the user will then zoom intothe POI for further examination with a higher resolution (e.g. 1 nm). Asan advantage of the present invention, the user will not need tomechanically move the specimen holder 9 to reposition or align thespecimen plane 8 to the center of a target small FOV 11. In other words,the specimen holder 9 remains stationary relative to the source 2 ofcharged particles, no matter the beam is scanning across a given largeFOV 10 or subsequently scanning across one, two or more small FOVs 11within that large FOV 10.

Instead, the user may run the second step by simply retrieving storeddeflecting parameter(s) of the macroscopic deflection sub-system 71which previously directed the beam to the center of the target small FOV11. The retrieved deflecting parameter(s) of the macroscopic deflectionsub-system 71 will then be re-applied to the subsystem 71, to direct thebeam to the center of the target small FOV 11. Generally, the positionof any small FOV within a large FOV may be controlled as desired by themacroscopic deflection sub-system 71 by retrieving and re-applyingstored deflecting parameters (e.g. voltage). As shown in FIG. 16, afterthe position of the small FOV within the large FOV is fixed by themacroscopic deflection sub-system 71, the retrieved and re-applieddeflecting parameter(s) of the macroscopic deflection sub-system 71 willremain unchanged. Then, the deflecting parameter(s) of the microscopicdeflection sub-system 72 is/are varied to cause the beam to scan acrossthe small FOV with a higher resolution.

In various embodiments of the invention, when the beam scans across thelarge FOV 10 in the first step, the spot F2 has a size A1. When the beamscans across the small FOV 11 within the large FOV 10 in the secondstep, the spot F2 has a size A2, and A2<A1. The inequation of A2<A1 willresult in the resolution for scanning a small FOV is higher than thatfor a large FOV.

Typically, the size of the large FOV 10 is adjustable, and its image mayrange from 50 um×50 um to 200 um×200 um in size with a resolution of0.5-20 nm. For example, the large FOV 10 may have a size of 100 um×100um with a resolution of 8 nm. The small FOV 11 (e.g. POI, or area ofinterest) is also adjustable, and it may range from 0.5 um×0.5 um to 5um×5 um in size with a resolution of 0.5-2 nm. For example, the smallFOV may have a size of 5 um×5 um with a resolution of 0.5 nm.

As shown in FIG. 17, the macroscopic deflection sub-system 71 mayinclude an upper deflector 71 a, and a lower deflector 71 b. Themicroscopic deflection sub-system 72 may be located between the upperdeflector 71 a and the lower deflector 71 b of the macroscopicdeflection system 71. The specimen holder 9 may be downstream withrespect to the lower deflector 71 b of the macroscopic deflectionsub-system 71. As shown in FIG. 18, the microscopic deflectionsub-system 72 may also include an upper deflector 72 a and a lowerdeflector 72 b.

Any other components known in any apparatus of charged-particle beam ortheir proper combination may be incorporated in the present invention.For a skilled person in the art, many of the components not shown inFIG. 10 are well-known, for example, suppressor electrode, beamextractor, anode, gun aperture, condenser lens that is responsible forprimary beam formation, beam blanker, stigmator for the correction ofasymmetrical beam distortions, objective aperture, SEM up detector,deflector, bright field (BF) detector, dark field (DF) detector. Asystem for the insertion into, motion within, and removal of specimensfrom the beam path is also needed. The system may include load lock,chamber interlock, lock port, loading and unloading mechanism, andtransfer table. Other parts in the microscope may be omitted or merelysuggested. In a specific yet exemplary electron microscope 1 as shown inFIG. 19, the source of charged particles may be an electron gun 2configured to emit an electron beam through gun aperture 12. Along thebeam trajectory, co-condenser 3 with magnetic coil 3C is placed betweengun aperture 12 and co-condenser 4 with a magnetic coil 4C. The electronbeam is focused to crossover spot F2 before it passes through beamblanking 13. After the beam passes through objective aperture 14, it isdeflected by an upper deflector 71 a and a lower deflector 71 b in themacroscopic deflection sub-system 71. It can also be deflected by anupper deflector 72 a and a lower deflector 72 b in the microscopicdeflection sub-system 72. In the meanwhile, the beam is focused by themagnetic objective lens 6 onto a specimen within the specimen holder 9.Electrons scattered from and penetrated through the specimen aredetected by the BSE detector 15, BF detector 16 and DF detector 17 forgenerating specimen images. Deflectors 71 a, 72 a, 72 b and 71 b mayreside in the central bore the magnetic objective lens 6, and they aredisk-shaped rings which are axially symmetric about the Z-axis. Eachdeflector may have a same or different diameter and may fit at aparticular position along the Z-axis. An actual bucket-shaped structuremay be used to holds the deflectors, and the structure is inserted intothe bucket-shaped space of the lens system thus making assembly easier.

The multiple deflection system (71 a, 71 b, 72 a and 72 b) is designedto control electron deflection with different FOV size. For example,deflectors or deflection nodes 71 a and 71 b control electron beam to beincident on a large FOV, while deflectors 72 a and 72 b on a small FOVsize.

The novel EM column system as shown in FIG. 19 can scan larger FOV withlow resolution (like 5, 10 or 20 nm) for the full FOV size. Then, the EMcolumn can switch to high resolution (like 1 nm) automatically withoutany position and focus change and start immediately to scan highresolution image on any special location. A specific software algorithmcan be used to control EM scanning of a larger FOV image with twodeflectors (71 a, 71 b) and co-condensers (3, 4) in a lower resolutionmode (i.e. a higher contribution from co-condenser 3 or lowercontribution from co-condenser 4). The algorithm will detect related POI(pattern of interesting) and record related location(s). As shown inFIG. 20, the algorithm can detect related POI (pattern of interesting)such as the morphological features of Covid-19 virus (SARS-CoV-2) in abiological sample and record their location(s). Then the software willswitch co-condensers (3, 4) to a higher resolution mode (i.e. a lowercontribution from co-condenser 3 or a higher contribution fromco-condenser 4). The two deflection nodes (71 a and 71 b) are set to orfixed to a controlled voltage. Other two deflection nodes (72 a and 72b) are then used to scan a small FOV 11 with the higher resolution. Asshown in the lower panel of FIG. 20, an image of Covid-19 virus(SARS-CoV-2) with a high resolution is acquired. A software system cancombine BSE, DF, BF's images from TEM/STEM system and use a machinelearning (ML) algorithm to generate an enhanced image with differencedimage resolution. Such operations, tasks, and functions are sometimesreferred to as being computer-executed, computerized,processor-executed, software-implemented, or computer-implemented. Theymay be realized by any number of hardware, software, and/or firmwarecomponents configured to perform the specified functions. For example,an embodiment of a system or a component may employ various integratedcircuit components, e.g., memory elements, digital signal processingelements, logic elements, look-up tables, or the like, which may carryout a variety of functions under the control of one or moremicroprocessors or other control devices.

When implemented in software or firmware, various elements of thesystems described herein are essentially the code segments or executableinstructions that, when executed by one or more processor devices, causethe host computing system to perform the various tasks. In certainembodiments, the program or code segments are stored in a tangibleprocessor-readable medium, which may include any medium that can storeor transfer information. Examples of suitable forms of non-transitoryand processor-readable media include an electronic circuit, asemiconductor memory device, a ROM, a flash memory, an erasable ROM(EROM), a floppy diskette, a CD-ROM, an optical disk, a hard disk, orthe like.

Through the above description of the embodiments, those skilled in theart can understand clearly that the present application may beimplemented by means of software plus necessary hardware platforms, orof course, may also be implemented all by software or hardware. Based onsuch understanding, the entirety of or a portion of that the technicalsolutions of the present application contribute over the background artmay be embodied in the form of a software product. The computer softwareproduct may be stored in storage medium, such as ROM/RAM, disk, opticaldisk, etc., and comprise several instructions for enabling one computerapparatus (which may be a personal computer, a server, or a networkequipment, etc.) to execute the methods described in the respectiveembodiments or described in certain parts of the embodiments of thepresent application.

In the foregoing specification, embodiments of the present inventionhave been described with reference to numerous specific details that mayvary from implementation to implementation. The specification anddrawings are, accordingly, to be regarded in an illustrative rather thana restrictive sense. The sole and exclusive indicator of the scope ofthe invention, and what is intended by the applicant to be the scope ofthe invention, is the literal and equivalent scope of the set of claimsthat issue from this application, in the specific form in which suchclaims issue, including any subsequent correction.

1. An apparatus of charged-particle beam comprising an objective lens, a sample stage, a BSE detector, a BF/DF detector, and a plasma generator; wherein the objective lens comprises an upper pole piece and a lower pole piece, wherein the sample stage is located between the upper pole piece and lower pole piece, wherein the BSE detector is located above the sample stage, wherein the BF/DF detector is located below the lower pole piece, and wherein the plasma generator is located between the lower pole piece and the BF/DF detector.
 2. The apparatus of charged-particle beam according to claim 1, wherein the plasma generator is so configured that it distributes or dissipates the plasma on surfaces of the BSE detector, the BF/DF detector, the lower pole piece and the sample stage in concentrations that are higher than that on surface of any other component(s) within the apparatus of charged-particle beam.
 3. The apparatus of charged-particle beam according to claim 2, wherein said other component(s) within the apparatus of charged-particle beam is a device of limiting aperture that is located above the objective lens.
 4. The apparatus of charged-particle beam according to claim 1, wherein the plasma generator is configured to periodically generate and distribute the plasma for selectively cleaning contaminants on the surface of the BSE detector and the BF/DF detector.
 5. The apparatus of charged-particle beam according to claim 1, wherein the BSE detector and the BF/DF detector have a voltage of 10-25V (e.g. negative voltage) to attract and concentrate the plasma onto their surface for more efficient and selective cleaning.
 6. The apparatus of charged-particle beam according to claim 1, wherein the plasma generator is configured not to block (or hinder) any charged-particles from irradiating on the BF/DF detector, wherein the plasma generator is configured not to block (or hinder) any charged-particles that would have been detected by the BF/DF detector in the absence of the plasma generator, or wherein the plasma generator is physically located outside trajectory of the charged-particles from the sample stage to the BF/DF detector.
 7. The apparatus of charged-particle beam according to claim 1, wherein the lower pole piece has a recess, and the plasma generator is partially or completely located within the recess.
 8. The apparatus of charged-particle beam according to claim 1, wherein the plasma is generated by the application of an electric field, a magnetic field, a microwave, or any combination thereof through a gas selected from oxygen, nitrogen, air, hydrogen, argon, helium, and neon.
 9. The apparatus of charged-particle beam according to claim 8, wherein the gas has a nearly vacuum pressure (<10 mTorr or 1 Pa) or a low or moderate pressure (< or ≈1 Torr or 100 Pa).
 10. The apparatus of charged-particle beam according to claim 1, wherein the plasma is selected from glow discharge plasma, capacitively coupled plasma (CCP), cascaded arc plasma, inductively coupled plasma (ICP), wave heated plasma, or any combinations thereof.
 11. The apparatus of charged-particle beam according to claim 1, wherein the plasma generator comprises a source of radio-frequency electrical power, a hollow cylindrical electrode formed of conducting material, and a grounded shield surrounding and enclosing the cylindrical electrode and electrically insulated therefrom; wherein the cylindrical electrode is in communication with the source of radio-frequency electrical power, and wherein, upon energizing the electrode with a radio-frequency electric power, a plasma is generated from gas in an interior of the cylindrical electrode.
 12. The apparatus of charged-particle beam according to claim 11, wherein a central hallow space (or the interior) of the cylindrical electrode is so positioned and dimensioned that it allows charged-particles to travel through it from the sample stage to the BF/DF detector, or it does not block (or hinder) any charged-particles from irradiating on the BF/DF detector, or it does not block (or hinder) any charged-particles that would have been detected by the BF/DF detector in the absence of the plasma generator.
 13. The apparatus of charged-particle beam according to claim 11, wherein the plasma generator further comprises a hollow dielectric cylinder formed of a dielectric material; wherein the cylindrical electrode is surrounding an exterior of the dielectric cylinder; and wherein, upon energizing the cylindrical electrode with radio-frequency electric power, a plasma is generated from gas in an interior of the dielectric cylinder by radio-frequency, hollow cathode effect coupling inside the dielectric cylinder.
 14. The apparatus of charged-particle beam according to claim 13, wherein a central hallow space (or the interior) of the dielectric cylinder is so positioned and dimensioned that it allows charged-particles to travel through it from the sample stage to the BF/DF detector, or it does not block (or hinder) any charged-particles from irradiating on the BF/DF detector, or it does not block (or hinder) any charged-particles that would have been detected by the BF/DF detector in the absence of the plasma generator.
 15. The apparatus of charged-particle beam according to claim 13, wherein the dielectric cylinder is formed of ceramic, glass, quartz, and Teflon such as a machinable ceramic comprising about 55% fluorophlogopite mica and 45% borosilicate glass.
 16. The apparatus of charged-particle beam according to claim 13, wherein a virtual anode is formed by the hollow cathode effect along a central axis of the dielectric cylinder in the plasma and a ground is defined by the BSE detector and/or the BF/DF detector.
 17. The apparatus of charged-particle beam according to claim 13, wherein the cylindrical electrode is a brass cylinder around an exterior diameter of the dielectric cylinder.
 18. The apparatus of charged-particle beam according to claim 13, further comprising a source of gas in fluid communication with the interior of the dielectric cylinder through a gas flow control device.
 19. The apparatus of charged-particle beam according to claim 1, which is an electron microscope (such as STEM), or an electron beam lithography apparatus.
 20. A method of selectively cleaning BSE detector and BF/DF detector in an apparatus of charged-particle beam, including installing a plasma generator within the apparatus as defined in claim 1, and generating plasma to selectively clean the BSE detector and/or the BF/DF detector. 